The present invention relates to an electric water heater, and more specifically to a control device for an electric water heater which drives heating elements to work alternatively subject to AC phase change, automatically turns off heating elements when no water passes through the water inlet, and has an overheat protection circuit to prevent an overheat.
A regular electric water heater, as shown in FIG. 1, comprises a water tank 30, four heating elements H1,H2,H3,H4 mounted in the water tank 30, and a thermostat 31, which controls the heating elements H1,H2,H3,H4 to heat water to the desired temperature. This structure of electric water heater is still not satisfactory in function. The drawbacks of this structure of electric water heater is outlined hereinafter.
1. When a low water temperature is required, only one heating element H1 is operated to heat water to the desired temperature. When a relatively higher temperature is required, two heating elements H1+H2, three heating elements H1+H2+H3 or four heating elements H1+H2+H3+H4 are turned on to heat water to the desired temperature. Normally, the first two heating elements H1,H2 are most frequently used. Therefore, the service life of the first two heating elements H1,H2 is relatively shorter than the other two heating elements H3,H4. When the first two heating elements H1,H2 are damaged, the last two heating elements H3,H4 may still function well. However the electric water heater becomes unable to function well when the first two heating elements H1,H2 are damaged. PA1 2. The thermostat controls the operation of the heating elements H1,H2,H3,H4 subject to the temperature of water being heated in the water tank 30. However, because the thermostat cannot detect the temperature of water at the water inlet of the water tank 30 and the temperature of water at the water outlet of the water tank 30, accurate temperature control cannot be achieved. PA1 3. This structure of electric water heater has no means to detect the flow rate of intake water, and water may be overheated or the electric water heater may explode when only a low flow rate of water is guided into the water tank 30 and the heating elements H1,H2,H3,H4 are continuously operated. PA1 4. This structure of electric water heater is not safe in use because it cannot automatically cut off power supply when one or a number of the heating elements H1,H2,H3,H4 are damaged.